Adaptive current reversal comparator

ABSTRACT

An adaptive current reversal comparator for synchronous switching regulator comparison circuit for use in a switching regulator is provided. The comparison circuit includes a voltage offset that is used at least in part to compensate for a propagation delay of the comparison circuit. The switching regulator includes an inductor and a synchronous transistor. The comparison circuit also includes a timing circuit that provides a threshold amount of time and an offset adjustment circuit. The offset adjustment circuit preferably decreases the voltage offset of the comparison circuit when a synchronous transistor control signal transitions to an OFF state more than the threshold amount of time prior to the discharge of an inductor. The comparison circuit also increases the voltage offset of the comparison circuit when the synchronous control signal transitions to an OFF state more than the threshold amount of time following the discharge of the inductor.

BACKGROUND OF THE INVENTION

This invention relates to voltage regulators. More specifically thisinvention relates to switching regulators with synchronousrectification.

A synchronous switching regulator typically includes a first transistorthat is ON during a first portion of a switching cycle and a secondtransistor that is ON during a second portion of the switching cycle.During the first portion of the switching cycle, the first transistorconducts between a power supply and an inductor. The second transistoris typically OFF during the first portion of the cycle. In the firstportion of the cycle, power is transmitted from the power supply throughthe first transistor to the inductor (which in turn is typically coupledto a load and/or load capacitor). During a second portion of the cycle,the second transistor, which may be referred to herein as thesynchronous transistor, turns ON and couples ground (or some othersuitable reference voltage) to the end of the inductor that is notcoupled to the load. In certain parts of the switching cycle, bothtransistors may be OFF and the end of the inductor not coupled to theload may, in fact, be floating.

Switching regulators with synchronous rectification preferably require acomparator, or for the purposes of this application other suitablecomparison circuit, that monitors current reversal across thesynchronous transistor. An ideal comparator with zero propagation delaywould simply trip—i.e., change from a first output state to a secondoutput state—when a sign reversal is detected across the synchronoustransistor. The output of the comparator is used to control theoperation of the second transistor.

However, real comparators must offset the trip point to compensate forthe propagation delays associated with the current reversal in theoutput inductor. The offset required depends on the propagation delay ofthe comparator and the time the inductor current takes to reach zerofrom the trip point. These two factors typically depend on the outputvoltage of the regulator and the inductor value used, both of which aretypically beyond the IC (integrated circuit) designer's control. Thecurrent state of the art is to select the most likely output voltage andits corresponding inductor value, and then calculate the offset voltagethe comparator needs. This approach works reasonably well for manyapplications. But for some applications the comparator either trips tooearly or too late, resulting in lowered efficiency.

It would be desirable to provide current reversal comparison circuitsand methods that automatically adjusts for any suitable output voltageand/or inductor value.

SUMMARY OF THE INVENTION

It is an object of the invention to provide current reversal comparisoncircuits and methods that are automatically adjustable for any suitableoutput voltage and/or inductor value.

A method of providing power to a load using a synchronous switchingregulator according to one embodiment of the invention is provided. Thesynchronous switching regulator includes a power supply, a maintransistor that typically operates as a switch, a synchronoustransistor, and an inductor having one side coupled to the load.

The method preferably includes turning ON a first transistor during afirst portion of a switching cycle. The first transistor is preferablycoupled to a second side of the inductor, the first transistor thatconducts between the power supply and the inductor during the firstportion of the switching cycle. The method also preferably includesturning ON a second transistor that is ON during a second portion of theswitching cycle and that is OFF during the remainder of the switchingcycle, the second transistor that is also coupled to the second side ofthe inductor. The second transistor conducts between ground and theinductor during the second portion of the switching cycle.

The method also includes detecting a trip point at which the voltage atthe second side of the inductor is about zero, the detecting using acomparison circuit, determining a time differential between the trippoint and an end of the second portion of the switching cycle, and,finally adjusting an offset of the comparison circuit in response to thetime differential.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the embodiments of the presentdisclosure can best be understood when read in conjunction with thefollowing drawings, in which the features are not necessarily drawn toscale but rather are drawn as to best illustrate the pertinent features,wherein

FIG. 1 is a synchronous switching regulator according to the invention;

FIG. 2 is a group of signal traces based on the circuit shown in FIG. 1;

FIG. 3 is a second group of signal traces based on the circuit in FIG.1;

FIG. 4 is a second synchronous switching regulator according to theinvention;

FIG. 5 is a group of signal traces based on the circuit in FIG. 4;

FIG. 6 is a second group of signal traces based on the circuit shown inFIG. 4;

FIG. 7 is a delay circuit; and

FIG. 8 is a group of signal traces based on the circuit shown in FIG. 7.

DESCRIPTION OF INVENTION

In the present invention, the offset of the current reversal comparatoris preferably automatically adjusted for any suitable output voltage andinductor value. The adaptive current reversal comparator ensures thesynchronous transistor is substantially always turned OFF at about zeroinductor current—i.e., about the point of current reversal in thesecondary switch.

In synchronous regulators, the current reversal comparator is activepreferably only in discontinuous conduction mode (DCM). The currentreversal comparator prevents inductor current reversal by turning OFFthe synchronous transistor thereby mimicking a rectifying diode toprevent the backflow of current through the inductor. In continuousconduction mode (CCM), the current reversal comparator is inactive,since the inductor current, by definition, does not reverse.

In convention implementations of DCM circuitry, the synchronoustransistor can either turn OFF too early or turn OFF too late. When itturns OFF too early, the stored energy left in the inductor pulls the SWpin 114 below ground, until the internal body diode of the synchronoustransistor conducts the necessary current and clamps the transistor pinat −0.7 volts. Typically, conducting through the body diode results in areduction of efficiency because it is more efficient to conduct throughthe channel of the synchronous transistor than it is to conduct throughthe body diode of the synchronous transistor. When the synchronoustransistor turns OFF too late, the reverse stored energy in the inductorpushes SW pin 114 HIGH right after the synchronous transistor is turnedOFF.

The adaptive current reversal comparator according to the inventiontakes advantage of these differences and servos its offset voltage suchthat the synchronous transistor is preferably turned OFF within apredetermined window of the discharge point of the inductor (wherein theinductor is substantially completely discharged)—i.e., the point ofcurrent reversal in the synchronous transistor. In other words, theoffset voltage of the adaptive current reversal comparator, whichcontrols where the comparator actually trips as opposed to where itshould trip, is servoed such that the comparator (which responds to thedischarge point of the inductor) substantially always trips within acertain predetermined time of the tripping of the synchronoustransistor.

Circuit Implementation

FIG. 1 shows the circuit implementation 100 of the adaptive currentreversal comparator (RCMP) 101. The offset of RCMP 101 is developed by acurrent dropped across resistor 102 (which is the offset resistorcoupled to the gate of transistor 135 and which receives a constantcurrent from current source 106.) This current is modulated bytransistor 104 to servo the offset voltage as needed. Transistor 104 isadapted to steer current away from a constant current source 106 asnecessary. Thus, the offset voltage can range from zero volts to amaximum of the current through source 106× the resistance of resistor102. Transistor 103 preferably provides level shifting to allow thecurrent steering to be implemented.

Charging and discharging capacitor 108 modulates the RCMP offsetvoltage. To increase the offset voltage, capacitor 108 is discharged,and conversely to decrease the offset voltage, capacitor 108 is charged.

Alternatively, the implementation of the comparator offset servo loopcould just as well be implemented with a digital counter (not shown) ora digital-to-analog converter (DAC) (also not shown) instead of usingcapacitor 108.

TDR 107 is typically controlled by an input from an oscillator. In DCM,BDR is controlled by the output of current reversal comparator 101.

In the case when the synchronous FET (NFET), transistor 110, is turnedOFF too early by BDR 109, the RCMP offset voltage should preferably bedecreased. (See FIG. 2, which includes trace 202 representing IL, thecurrent through the inductor, trace 204 which represents the SW pin 114that is coupled to inductor 112, trace 206 which represents BDR 109input, trace 208 which represents RESETB which is input to latches 117and 119, trace 210 PG, which shows implementation of the decrease of theoffset voltage.¹ The stored energy left in inductor 112 pushes SW pin114 (which is located at the second end of the inductor) below groundright after BDR 109 goes LOW. Some time after the inductor current hasdischarged SW pin 114 swings HIGH and settles to the output voltage(VOUT). The time it takes BDR 109 (which responds to current reversal intransistor 110 as stated above) to go LOW and SW 114 to go HIGH iscompared against a 20 nanoseconds delayed BDR signal 115 (which iscoupled to receive an input from BDR 109). If the time differentialbetween BDR 109 going LOW and SW 114 going ¹ Trace 212 does notoscillate in FIG. 2 but its equivalent trace 312 in FIG. 3 doesoscillate. Trace 312 NG implements the increase of the offset voltage(see FIG. 3) wherein the BDR trace 306 shows BDR 109 forcing transistor110 LOW after inductor 112, carrying IL 202, has fully discharged andreversed polarity. HIGH is greater than 20 nanoseconds, NOR gate 116 andNAND gate 118 preferably latch (which are implemented via logic 117 and119, respectively, and that have outputs that are inverted usinginverters 133 and 132, respectively), and drive PG 120 (coupled to thegate of transistor 122, transistor 122 having a source coupled toconstant current source 138) LOW, thereby charging capacitor 108 (seetrace 210 in FIG. 2). Block 121 preferably provides a delay of somepredetermined amount—e.g., 70 nanoseconds—to allow transistor 104 tocharge up sufficiently so as to be able to influence the operation ofcurrent reversal comparator 101.

In the situation when transistor 110 is turned OFF too late, SW pin 114swings HIGH immediately after BDR 109 goes LOW. In this case gate 116and gate 118 latch in the opposite state, and drive NG 123 (which iscoupled to the gate of transistor 124 which itself is preferably coupledto constant current sink 136) is HIGH thereby discharging capacitor 108(see trace 310 in FIG. 3). FIG. 3 includes trace 302 representing IL,the current through the inductor, trace 304 which represents the pinvoltage at SW pin 114, trace 306 which represents BDR 109 input, trace308 which represents RESETB which is input to latches 117 and 119, trace309 PG (which is flat under the conditions shown in FIG. 3), and trace310 which shows NG implementing the increase of the offset voltage.

CCM at High Load and CCM at Low Load

Additional circuitry may be needed to recognize and/or distinguish thesituation where the regulator is running in CCM at relatively HIGHoutput load current, and CCM at relatively LOW output load current. InCCM at HIGH output load current (the traces for which are shown in FIG.5), the inductor current does not reverse—i.e., the inductor current istypically above a certain threshold value. Thus, circuitry is preferablyonly needed to prevent capacitor 108 from charging or discharging—i.e.,circuitry that maintains capacitor 108 at a preferably predeterminedindependent value.

In CCM at LOW output load current (the traces for which are shown inFIG. 6), the regulator may be running in forced continuous mode and theinductor current may reverse. In this case, capacitor 108 needs todischarge to raise the RCMP offset voltage.

FIG. 4 shows additional circuitry. This additional circuitry preferablydistinguishes CCM at HIGH output load current from CCM at LOW outputload current (and, preferably, both conditions from DCM mode). Todistinguish CCM at HIGH output load current from CCM at LOW output loadcurrent, NICMP comparison circuit 450 is used to detect whether SW pin114 dips below ground or not. In the case of CCM at HIGH output loadcurrent, SW pin 114 dips below ground immediately following the fallingedge of BDR 109 and circuit 450 goes HIGH. (See FIG. 5 wherein trace 502shows the inductor current IL 112, trace 504 shows SW pin 114, trace 506shows BDR 109, trace 508 shows TDR 107, and trace 510 shows circuit450.) In CCM at HIGH output load current, circuit 450 resets latch 452,which also is coupled to receive an input from TDR 107, allows TDR 107through latch 454 and gate 456 to pull RESETB LOW (see trace 512),keeping both PG 120 HIGH and NG 123 LOW (see traces 514 and 516).Keeping PG 120 HIGH and NG 123 LOW maintains the offset voltageindependent of oscillation of IL 502 at preferably more than apredetermined threshold.

In the case of CCM at LOW output load current, SW pin 114 does not dipbelow ground immediately following the falling edge of BDR 109 andcircuit 450 stays LOW. (See FIG. 6 wherein trace 602 shows inductorcurrent 112, trace 604 shows SW pin 114, trace 606 shows BDR 109, trace608 shows TDR 107, trace 609 shows NICMP, trace 610 shows RESETB, trace612 shows NG 123, and trace 614 shows PG 120.) In this situation, latch452 preferably does not reset which prevents TDR 107 from going throughlatch 454. This results in a circuit that behaves substantiallyidentically to the circuit in FIG. 1. capacitor 108 is allowed to chargeor discharge—i.e., is flexible—as needed to prevent the inductor currentfrom reversing. Inverter 481 and 482 invert the signals at the outputsof TDR 107 and 450 respectively.

The embodiments described hereinabove are further intended to explainbest modes known of practicing the invention and to enable othersskilled in the art to utilize the invention in such, or other,embodiments and with the various modifications required by theparticular applications or uses of the invention. Accordingly, thedescription is not intended to limit the invention to the form disclosedherein. Also, it is intended that the appended claims be construed toinclude alternative embodiments.

1. A method for adjusting the offset of a synchronous switchingregulator comparison circuit, the synchronous switching regulatorcomparison circuit for use in a switching regulator, the comparisoncircuit comprising a voltage offset that is used at least in part tocompensate for a propagation delay of the comparison circuit, theswitching regulator comprising an inductor and a synchronous transistor,the operation of the synchronous transistor controlled by a synchronoustransistor control signal, the method comprising: comparing thesynchronous transistor control signal with the discharge of theinductor; when the synchronous transistor control signal transitions toan OFF state more than a threshold amount of time prior to the dischargeof the inductor, decreasing the offset of the synchronous switchingregulator comparison circuit; and when the synchronous control signaltransitions to an OFF state more than a threshold amount of time afterthe discharge of the inductor, increasing the offset.
 2. The method ofclaim 1 further comprising determining whether the synchronoustransistor control signal transitions to an OFF state more than athreshold amount of time prior to the discharge of the inductor.
 3. Themethod of claim 1 further comprising determining whether the switchingregulator is operating in discontinuous conduction mode or continuousconduction mode.
 4. The method of claim 1 further comprising determiningwhether the switching regulator is operating in a continuous conductionmode at a light output current load characterized by a transistor pinthat does not drop below ground.
 5. The method of claim 1 furthercomprising determining whether the switching regulator is operating in acontinuous conduction mode at a relatively heavy output current modecharacterized by a pin coupled to the synchronous transistor thatperiodically drops below ground.
 6. A synchronous switching regulatorcomparison circuit for use in a switching regulator, the comparisoncircuit comprising a voltage offset that is used at least in part tocompensate for a propagation delay of the comparison circuit, theswitching regulator comprising an inductor and a synchronous transistor,the comparison circuit comprising: a timing circuit that provides athreshold amount of time; and an offset adjustment circuit thatdecreases the voltage offset of the comparison circuit when asynchronous transistor control signal transitions to an OFF state morethan the threshold amount of time prior to the discharge of an inductorand increases the voltage offset of the comparison circuit when thesynchronous transistor control signal transitions to an OFF state morethan the threshold amount of time following the discharge of theinductor.
 7. The comparison circuit of claim 6 further comprising adelay circuit that provides a delay that corresponds to the thresholdamount of time.
 8. The comparison circuit of claim 6 further comprisinga logic circuit that determines whether the switching regulator isoperating in discontinuous conduction mode or continuous conductionmode.
 9. The comparison circuit of claim 6 further comprising a logiccircuit that determines whether the switching regulator is operating ina continuous conduction mode at a light output current loadcharacterized by a synchronous transistor pin that does not drop belowreference voltage level.
 10. The comparison circuit of claim 6 furthercomprising a logic circuit that determines whether the switchingregulator is operating in a continuous conduction mode at a heavy outputcurrent mode characterized by a synchronous transistor pin thatperiodically drops below a reference voltage level.
 11. A synchronousswitching regulator comprising: a power supply; an inductor having afirst side coupled to a load; a first transistor that is ON during afirst portion of a switching cycle, the first transistor coupled to asecond side of the inductor; a second transistor that is ON during asecond portion of the switching cycle and that is OFF during theremainder of the switching cycle, the second transistor that is coupledto the second side of the inductor; the first transistor that conductsbetween the power supply and the load during the first portion of theswitching cycle; the second transistor that conducts between ground andthe inductor during the second portion of the switching cycle; asynchronous switching regulator comparator circuit that detects a trippoint at which the voltage at the second side of the inductor is aboutzero; a measurement circuit that determines a time differential betweenthe trip point and an end of the second portion of the switching cycle;and an offset adjustment circuit that adjusts an offset of thecomparison circuit in response to a determination of the measurementcircuit, the offset of the comparison circuit that controls a magnitudeof the time difference between the total discharge of the inductor andthe trip point of the comparator circuit.
 12. The switching regulator ofclaim 11 further comprising a differentiation circuit that determineswhether the switching regulator is operating in a continuous conductionmode characterized by zero current reversals or operating in acontinuous conduction mode characterized by periodic current reversals.13. The switching regulator of claim 11, the offset adjustment circuitfurther operable to decrease the offset of the comparison circuit whenthe measurement circuit determines that the trip point occurs more thana predetermined amount of time before the end of the second portion ofthe switching cycle.
 14. The switching regulator of claim 11, the offsetadjustment circuit further operable to increase the offset of thecomparison circuit when the measurement circuit determines that the trippoint occurs more than a predetermined amount of time after the end ofthe second portion of the switching cycle.
 15. A method of providingpower to a load using a synchronous switching regulator, the synchronousswitching regulator comprising a first transistor, a synchronoustransistor, a power supply and an inductor having a first side coupledto a load, the method comprising: turning ON the first transistor duringa first portion of a switching cycle, the first transistor coupled to asecond side of the inductor, the first transistor that conducts betweenthe power supply and the inductor during the first portion of theswitching cycle; turning ON a second transistor, the second transistorthat is ON during a second portion of the switching cycle and that isOFF during the remainder of the switching cycle, the second transistorthat is coupled to the second side of the inductor, the secondtransistor that conducts between ground and the inductor during thesecond portion of the switching cycle; detecting a trip point at whichthe voltage at the second side of the inductor is about zero, thedetecting using a comparison circuit; determining a time differentialbetween the trip point and an end of the second portion of the switchingcycle; and adjusting an offset of the comparison circuit in response tothe time differential.
 16. The method of claim 15 further comprisingdetermining whether the switching regulator is operating in a continuousconduction mode characterized by zero current reversals or operating ina continuous conduction mode characterized by periodic currentreversals.
 17. The method of claim 15, further comprising decreasing theoffset of the comparison circuit when the trip point occurs more than apredetermined amount of time before the end of the second portion of theswitching cycle.
 18. The method of claim 15, further comprisingincreasing the offset of the comparison circuit when the trip pointoccurs more than a predetermined amount of time after the end of thesecond portion of the switching cycle.
 19. A synchronous switchingregulator comparison circuit for use in a switching regulator, thecomparison circuit comprising a voltage offset that is used at least inpart to compensate for a propagation delay of the comparison circuit,the switching regulator comprising an inductor and a synchronoustransistor, the comparison circuit comprising: a delay circuit thatprovides a threshold amount of delay; a first offset adjustment loopthat decreases the voltage offset of the comparison circuit when asynchronous transistor control signal transitions to an OFF state morethan the threshold amount of delay prior to the discharge of aninductor; and a second offset adjustment loop that increases the voltageoffset of the comparison circuit when the synchronous control signaltransitions to an OFF state more than the threshold amount of delayfollowing the discharge of the inductor.